The invention relates to a DNA encoding a polypeptide with 1-deoxy-D-xylulose-5-phosphate reductoisomerase (DXPRI) activity which originates from plants. In addition, the invention relates to the use of DNA sequences encoding a polypeptide with DXPRI activity which originates from plants for the generation of plants with an elevated tocopherol, carotenoid, vitamin K, chlorophyll and polyterpene content, specifically to the use of the DNA sequence SEQ ID No. 1 or of DNA sequences hybridizing herewith, to a method for the generation of plants with an elevated tocopherol, carotenoid, vitamin K, chlorophyll and polyterpene content, and to the resulting plant itself.
The generation of plants with an elevated sugar, enzyme and amino acid content has hitherto been an important objective in plant molecular genetics. The development of plants with an elevated vitamin content, such as, for example, an elevated tocopherol content, is, however, also of economic interest.
The naturally occurring eight compounds with vitamin E activity are derivatives of 6-chromanol (Ullmann""s Encyclopedia of Industrial Chemistry, Vol. A 27 (1996), VCH Verlagsgesellschaft, Chapter 4., 478-488, Vitamin E). The first group (1a-d) is derived from tocopherol, while the second group is composed of tocotrienol derivatives (2a-d): 
1a, xcex1-tocopherol: R1xe2x95x90R2xe2x95x90R3=CH3 
1b, xcex2-tocopherol [148-03-8]: R1xe2x95x90R3=CH3, R2=H
1c, xcex3-tocopherol [54-28-4]: R1=H, R2xe2x95x90R3=CH3 
1d, xcex4-tocopherol [119-13-1]: R1xe2x95x90R2=H, R3 =CH3
2a, xcex1-tocotrienol [1721-51-3]: R1xe2x95x90R2xe2x95x90R3=CH3 
2b, xcex2-tocotrienol [490-23-3]: R1xe2x95x90R3=CH3, R2=H
2c, xcex3-tocotrienol [14101-61-2]: R1=H, R2xe2x95x90R3=CH3 
2d, xcex4-tocotrienol [25612-59-3]: R1xe2x95x90R2=H, R3=CH3 
xcex1-Tocopherol has great economic importance.
The development of crop plants with an elevated tocopherol, vitamin K, carotenoid, chlorophyll and polyterpene content by means of tissue culture or seed mutagenesis and natural selection is set a limit. Thus, on the one hand, it must be possible to appraise, for example, the tocopherol content, or content of the desired catabolite, as early as in the tissue culture stage, and, on the other hand, only those plants whose regeneration from cell cultures to entire plants is successful can be manipulated by tissue culture techniques. Also, crop plants may show undesirable characteristics after mutagenesis and selection, and these characteristics must be reeliminated by, in some cases repeated, back crosses. Also, for example, the increase in tocopherol content would be restricted to crosses between plants of the same species.
This is why the genetic engineering approach of isolating essential biosynthesis genes which encode, for example, tocopherol synthesis performance and introducing them into crop plants in a directed fashion is superior to the traditional breeding method. Knowledge of the biosynthesis and its regulation, and identification of genes which affect biosynthesis performance, are prerequisites for this method.
Isoprenoids or terpenoids are composed of a variety of classes of lipid-soluble molecules, and they are formed partially or exclusively from C5-isoprene units. Pure prenyl lipids (for example carotenoids) are composed of C skeletons based exclusively on isoprene units, while mixed prenyl lipids (for example chlorophylls, tocopherols and vitamin K), have an isoprenoid side chain linked to an aromatic nucleus.
The biosynthesis of prenyl lipids starts with 3xc3x97acetyl-CoA units which are converted into the starting isoprene unit (C5), namely isopentenyl pyrophosphate (IPP), via xcex2-hydroxymethylglutaryl-CoA (HMG-CoA) and mevalonate. Recent C13 in vivo feeding experiments have demonstrated that the IPP formation pathway in various eubacteria, green algae and plant chloroplasts is mevalonate-independent (FIG. 1). In this pathway, hydroxyethylthiamine, which is formed by decarboxylation of pyruvate, and glycerolaldehyde-3-phosphate (3-GAP) are first converted into 1-deoxy-D-xylulose-5-phosphate in a xe2x80x9ctransketolasexe2x80x9d reaction mediated by 1-deoxy-D-xylulose-5-phosphate synthase (DOXS) (Lange et al., 1998; Schwender et al., 1997; Arigoni et al., 1997; Lichtenthaler et al., 1997; Sprenger et al., 1997). In an intramolecular rearrangement reaction, this 1-deoxy-D-xylulose-5-phosphate is converted by DXPRI into 2-C-methyl-D-erythritol-4-phosphate and then into IPP (Arigoni et al., 1997; Zeidler et al., 1998). Biochemical data suggest that the mevalonate pathway operates in the cytosol and leads to the formation of phytosterols. The antibiotic mevinolin, a specific mevalonate formation inhibitor, only leads to sterol biosynthesis inhibition in the cytoplasma, while prenyl lipid formation in the plastids remains unaffected (Bach and Lichtenthaler, 1993). In contrast, the mevalonate-independent pathway is located in the plastids and leads predominantly to the formation of carotenoids and plastid prenyl lipids (Schwender et al., 1997; Arigoni et al., 1997).
IPP is in equilibrium with its isomer, dimethylallyl pyrophosphate (DMAPP). Condensation of IPP with DMAPP head to tail results in the monoterpene (C10) geranylpyrophosphate (GPP). Addition of further IPP units results in the sesquiterpene (C15) farnesyl pyrophosphate (FPP), and to the diterpene (C20) geranylgeranyl pyrophosphate (GGPP). Bonding between two GGPP molecules results in the formation of the C40 precursors of carotenoids.
In the case of mixed prenyl lipids, the isoprene side chain, whose length varies, is linked to non-isoprene rings such as, for example, a porphyrine ring in the case of chlorophylls a and b. The chlorophylls and phylloquinones contain a C20 phytyl chain, in which only the first isoprene unit contains a double bond. GGPP is converted by geranylgeranyl pyrophosphate oxidoreductase (GGPPOR) to give phytyl pyrophosphate (PPP), the starting material for the subsequent formation of tocopherols.
The ring structures of the mixed prenyl lipids which lead to the formation of vitamins E and K are quinones whose starting metabolites are derived from the shikimate pathway. The aromatic amino acids phenylalanine or tyrosine are converted into hydroxyphenyl pyruvate, which is dioxygenated to give homogentisic acid. The chorismate is formed, on the one hand, via erythrose-4-phosphate, 3xe2x80x2-dehydroquinate, 3xe2x80x2-dehydroshikimate, shikimate, shikimate-3-phosphate and 5xe2x80x2-enolpyruvylshikimate-3-phosphate (FIG. 1). In this process, fructose-6-phosphate and glycerolaldehyde-3-phosphate are reacted to give xylulose-5-phosphate and erythrose-4-phosphate. The above-described homogentisic acid is subsequently bonded to PPP to form the precursor of xcex1-tocopherol and xcex1-tocoquinone, namely 2-methyl-6-phytylquinol. Methylation steps with S-adenosylmethionine as methyl group donor lead first to 2,3-dimethyl-6-phytylquinol, subsequent cyclization leads to xcex3-tocopherol and further methylation to xcex1-tocopherol (Richter, Biochemie der Pflanzen [Plant biochemistry], Georg Thieme Verlag Stuttgart, 1996).
Examples which demonstrate that manipulation of an enzyme may directionally affect metabolite flow can be found in the literature. A direct effect on the quantities of carotenoids in these transgenic tomato plants was measured in experiments on an altered expression of phytoene synthase, which links two GGPP molecules to give 15-cis-phytoene (Fray and Grierson, Plant Mol. Biol. 22(4), 589-602 (1993); Fray et al., Plant J., 8, 693-701 (1995)). As expected, transgenic tobacco plants which have reduced quantities of phenylalanine-ammonium lyase show reduced quantities of phenylpropanoid. The enzyme phenylalanine-ammonium lyase catalyzes the degradation of phenylalanine and thus withdraws it from phenylpropanoid biosynthesis (Bate et al., Proc. Natl. Acad. Sci USA 91 (16): (1994) 7608-7612; Howles et al., Plant Physiol. 112. (1996) 1617-1624).
Little is known to date on increasing the metabolite flow for elevating the tocopherol content in plants by overexpression of individual biosynthesis genes. Only WO 97/27285 describes a modification of the tocopherol content by stronger expression or down-regulation of the enzyme p-hydroxyphenylpyruvate dioxygenase (HPPD).
It is an object of the present invention to develop a transgenic plant with an elevated tocopherol, vitamin K, carotenoid, chlorophyll and polyterpene content.
We have found that this object has been achieved by overexpressing a 1-deoxy-D-xylulose-5-phosphate reductoisomerase (DXPRI) gene in the plants.
To increase the metabolite flow from the primary metabolism into the tocopherol biosynthesis, for example, the formation of 2-C-methyl-D-erythritol-4-P as essential starting substrate for all plastid isoprenoids was increased. To this end, DXPRI activity in transgenic plants was increased by overexpressing the Arabidopsis thaliana DXPRI gene. In principle, this can also be achieved by expressing homologous or heterologous DXPRI genes. A nucleotide sequence encoding a DXPRI was described for E.coli (Accession Number AB 013300; Kuzuyama et al., 1998; Takahashi et al., 1998).
Example 1 describes for the first time a plant DXPRI gene (FIG. 2, SEQ-ID No. 1) from Arabidopsis thaliana, which is expressed at a higher level in transgenic plants. To ensure localization in the plastids, a transit signal sequence (FIG. 3, FIG. 4) is arranged upstream of the Arabidopsis thaliana DXPRI nucleotide sequence. Fragment A (529 bp) in FIG. 4 contains the cauliflower mosaic virus 35S promoter (nucleotides 6909 to 7437 of the cauliflower mosaic virus). Fragment B (259 bp) contains the transketolase transit peptide. Fragment E contains the DXPRI gene. Fragment D (192 bp) contains the polyadenylation signal of gene 3 of the Ti-plasmid pTIACH5 T-DNA (Gielen et al., 1984) to terminate transcription. Another suitable expression cassette is a DNA sequence which encodes a DXPRI gene which hybridizes with a SEQ ID No. 1 and which is derived from other organisms or other plants.
The 2-C-methyl-D-erythritol-4-P, of which greater quantities are now available owing to the additional expression of the DXPRI gene, is reacted further towards tocopherols, carotenoids, vitamin K, chlorophylls and polyterpenes.
The transgenic plants are generated by transforming the plants with a construct comprising the DXPRI gene. Tobacco and oilseed rape were used as model, plants for the production of tocopherols, vitamin K, carotenoids, chlorophylls and polyterpenes.
Antisense constructs and homologous or heterologous plant DXPRI genes were transformed independently of one another into plants (FIG. 5). Fragment A (529 bp) in FIG. 5 contains the cauliflower mosaic virus 35S promoter (nucleotides 6909 to 7437 of the cauliflower mosaic virus). Fragment B (259 bp) contains the transketolase transit peptide (FIG. 3). Fragment E contains the DXPRI gene in antisense orientation. Fragment D (192 bp) contains the polyadenylation signal of gene 3 of the Ti-plasmid pTIACH5 T-DNA (Gielen et al., 1984) to terminate transcription. Measurements on DXPRI antisense plants showed a drastic decrease in tocopherol and carotenoid contents. This confirms the direct effect of the plastid plant DXPRI on carotenoid and tocopherol synthesis.
The invention relates to the use of the Arabidopsis thaliana DNA sequence SEQ ID No. 1 which encodes a DXPRI or its functional equivalents for the generation of a plant with an elevated tocopherol, carotenoid, vitamin K, chlorophyll and polyterpene content. The nucleic acid sequence may be, for example, a DNA or cDNA sequence. Encoding sequences which are suitable for insertion into an expression cassette are, for example, those which encode a DXPRI and which allow the host to overproduce tocopherols, carotenoids, vitamin K, chlorophylls and polyterpenes.
The expression cassettes also comprise regulative nucleic acid sequences which govern the expression of the encoding sequence in the host cell. In a preferred embodiment, an expression cassette comprises a promoter upstream, i.e. on the 5xe2x80x2-end of the encoding sequence, and a polyadenylation signal downstream, i.e. on the 3xe2x80x2-end, and, if appropriate, further regulatory elements which are linked operatively with the sequence in between which encodes the DXPRI gene. Operative linkage is to be understood as meaning the sequential arrangement of promoter, encoding sequence, terminator and, if appropriate, further regulatory elements in such a way that each of the regulatory elements can fulfil its function as intended when the encoding sequence is expressed. The sequences preferred for operative linkage, but not restricted thereto, are targeting sequences for guaranteeing subcellular localization in the apoplast, in the vacuole, in plastids, in the mitochondrion, in the endoplasmatic reticulum (ER), in the nucleus, in oil bodies or in other compartments, and translation enhancers such as the tobacco mosaic virus 5xe2x80x2-leader sequence (Gallie et al., Nucl. Acids Res. 15 (1987), 8693-8711).
As an example, the plant expression cassette can be incorporated into the tobacco transformation vector pBinAR-Hyg. FIG. 6 shows the tobacco transformation vectors pBinAR-Hyg with 35S promoter (A) and pBinAR-Hyg with the seed-specific promoter phaseolin 796 (B):
HPT: hygromycin phosphotransferase
OCS: octopin synthase terminator
PNOS: nopalin synthase promoter
also shown are those restriction cleavage sites which cut the vector only once.
A suitable promoter of the expression cassette is, in principle, any promoter which is capable of governing the expression of foreign genes in plants. In particular, a plant promoter or a promoter derived from a plant virus is preferably used. Particularly preferred is the CaMV 35S promoter from cauliflower mosaic virus (Franck et al., Cell 21 (1980), 285-294). As is known, this promoter contains various recognition sequences for transcriptional effectors which in their totality lead to permanent and constitutive expression of the introduced gene (Benfey et al., EMBO J. 8 (1989), 2195-2202).
The expression cassette may also comprise a chemically inducible promoter which allows expression of the exogenous DXPRI gene in the plant to be governed at a particular point in time. Such promoters which can be used are, inter alia, for example the PRP1 promoter (Ward et al., Plant. Mol. Biol. 22 (1993), 361-366), a salicylic-acid-inducible promoter (WO 95/19443), a benzenesulfonamide-inducible promoter (EP-A 388186), a tetracyclin-inducible promoter (Gatz et al., (1992) Plant J. 2, 397-404), an abscisic-acid-inducible promoter (EP-A 335528) or an ethanol- or cyclohexanone-inducible promoter (WO 93/21334).
Furthermore, particularly preferred promoters are those which ensure expression in tissues or parts of the plant in which, for example, the biosynthesis of tocopherol or its precursors takes place. Promoters which ensure leaf-specific expression must be mentioned in particular. Promoters which must be mentioned are the potato cytosolic FBPase or the potato ST-LSI promoter (Stockhaus et al., EMBO J. 8 (1989), 2445-245).
A foreign protein was expressed stably in the seeds of transgenic tobacco plants to an extent of 0.67% of the total soluble seed protein with the aid of a seed-specific promoter (Fiedler and Conrad, Bio/Technology 10 (1995), 1090-1094). The expression cassette can therefore contain, for example, a seed-specific promoter (preferably the phaseolin promoter (U.S. Pat. No. 5,504,200), the USP promoter (Baumlein, H. et al., Mol. Gen. Genet. (1991) 225 (3), 459-467) or the LEB4 promoter (Fiedler and Conrad, 1995)), the LEB4 signal peptide, the gene to be expressed and an ER retention signal.
An expression cassette is generated by fusing a suitable promoter with a suitable DXPRI DNA sequence and, preferably, a DNA which is inserted between promoter and DXPRI DNA sequence and which encodes a chloroplast-specific transit peptide, and with a polyadenylation signal, using customary recombination and cloning techniques as they are described, for example, by T. Maniatis, E. F. Fritsch and J. Sambrook, Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y. (1989) and by T. J. Silhavy, M. L. Berman and L. W. Enquist, Experiments with Gene Fusions, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y. (1984) and by Ausubel, F. M. et al., Current Protocols in Molecular Biology, Greene Publishing Assoc. and Wiley-Interscience (1987).
Particularly preferred sequences are those which ensure targeting into the apoplast, into plastids, into the vacuole, into the mitochondrion or into the endoplasmatic reticulum (ER) or which, due to a lack of suitable operative sequences, ensure that the product remains in the compartment of formation, the cytosol (Kermode, Crit. Rev. Plant Sci. 15, 4 (1996), 285-423). Localization in the ER has proved to be particularly advantageous for the amount of protein accumulation in transgenic plants (Schouten et al., Plant Mol. Biol. 30 (1996), 781-792).
Other expression cassettes which can be used are those whose DNA sequence encodes a DXPRI fusion protein, part of the fusion protein being a transit peptide which governs translocation of the polypeptide. Chloroplast-specific transit peptides which are cleaved off enzymatically from the DXPRI residue after translocation of the DXPRI gene into the chloroplasts are preferred. Particularly preferred is the transit peptide derived from plastid DXPRI or from a functional equivalent of this transit peptide (for example the transit peptide of the Rubisco small subunit or of ferredoxin NADP oxidoreductase).
Especially preferred are DNA sequences of three cassettes of the plastid transit peptide of potato plastid transketolase in three reading frames as KpnI/BamHI fragments with an ATG codon in the NcoI cleavage site:
pTP09
KpnI_GGTACCATGGCGTCTTCTTCTTCTCTCACTCTCTCTCAAGCTATCCTCTCTCGTTCTGTC CCTCGCCATGGCTCTGCCTCTTCTTCTCAACTTTCCCCTTCTTCTCTCACTTTTTCCGGCCTTAA ATCCAATCCCAATATCACCACCTCCCGCCGCCGTACTCCTTCCTCCGCCGCCGCCGCCGCCGTCG TAAGGTCACCGGCGATTCGTGCCTCAGCTGCAACCGAAACCATAGAGAAAACTGAGACTGCGGGA TCC_BamHI
pTP10
KpnI_GGTACCATGGCGTCTTCTTCTTCTCTCACTCTCTCTCAAGCTATCCTCTCTCGTTCTGTC CCTCGCCATGGCTCTGCCTCTTCTTCTCAACTTTCCCCTTCTTCTCTCACTTTTTCCGGCCTTAA ATCCAATCCCAATATCACCACCTCCCGCCGCCGTACTCCTTCCTCCGCCGCCGCCGCCGCCGTCG TAAGGTCACCGGCGATTCGTGCCTCAGCTGCAACCGAAACCATAGAGAAAACTGAGACTGCGCTG GATCC_BamHI
pTP11
KpnI_GGTACCATGGCGTCTTCTTCTTCTCTCACTCTCTCTCAAGCTATCCTCTCTCGTTCTGTC CCTCGCCATGGCTCTGCCTCTTCTTCTCAACTTTCCCCTTCTTCTCTCACTTTTTCCGGCCTTAA ATCCAATCCCAATATCACCACCTCCCGCCGCCGTACTCCTTCCTCCGCCGCCGCCGCCGCCGTCG TAAGGTCACCGGCGATTCGTGCCTCAGCTGCAACCGAAACCATAGAGAAAACTGAGACTGCGGGG ATCC_BamHI
The inserted nucleotide sequence encoding a DXPRI can be prepared synthetically, obtained naturally or contain a mixture of synthetic and natural DNA constituents, and may be composed of various heterologous DXPRI gene segments of a variety of organisms. In general, synthetic nucleotide sequences are produced which are equipped with codons which are preferred by plants. These codons which are preferred by plants can be determined from codons with the highest protein frequency expressed in the plant species of the highest interest. When preparing an expression cassette, a variety of DNA fragments may be manipulated in order to obtain a nucleotide sequence which expediently reads in the correct direction and which is equipped with a correct reading frame. Adaptors or linkers may be added to the fragments in order to link the DNA fragments to each other.
The promoter and terminator regions may expediently be provided, in the direction of transcription, with a linker or polylinker containing one or more restriction sites for insertion of this sequence. As a rule, the linker has 1 to 10, in most cases 1 to 8, preferably 2 to 6, restriction sites. In general, the linker within the regulatory regions has a size less than 100 bp, frequently less than 60 bp, but at least 5 bp. The promoter may be native, or homologous, or else foreign, or heterologous, to the host plant. The expression cassette comprises, in the 5xe2x80x2-3xe2x80x2 direction of transcription, the promoter, a DNA sequence encoding a DXPRI gene, and a region for transcriptional termination. Various termination regions may be exchanged for each other as desired.
Manipulations which provide suitable restriction cleavage sites or which eliminate the excess DNA or restriction cleavage sites may also be employed. In vitro mutagenesis, primer repair, restriction or ligation may be used in cases where insertions, deletions or substitutions such as, for example, transitions and transversions, are suitable. Complementary ends of the fragments may be provided for ligation in the case of suitable manipulations such as, for example, restriction, chewing back or filling in overhangs for blunt ends.
A procedure which may be of importance for the success according to the invention may be, inter alia, the attachment of the specific ER retention signal SEKDEL (Schouten, A. et al., Plant Mol. Biol. 30 (1996), 781-792), thus tripling to quadrupling the average expression level. Other retention signals which occur naturally in plant and animal proteins localized in the ER may also be employed for constructing the cassette.
Preferred polyadenylation signals are plant polyadenylation signals, preferably those which correspond essentially to Agrobacterium tumefaciens T-DNA-polyadenylation signals, in particular those of gene 3 of the T-DNA (octopin synthase) of the Ti-plasmid pTiACH5 (Gielen et al., EMBO J. 3 (1984), 835 et seq.), or functional equivalents.
For example, an expression cassette may comprise a constitutive promoter (preferably the CaMV 35 S promoter), the LeB4 signal peptide, the gene to be expressed, and the ER retention signal. The preferred ER retention signal used is the amino acid sequence KDEL (lysine, aspartic acid, glutamic acid, leucine).
The fused expression cassette which encodes a DXPRI gene is preferably cloned into a vector, for example pBin19, which is suitable for transforming Agrobacterium tumefaciens. Agrobacteria transformed with such a vector can then be used in a known manner for transforming plants, in particular crop plants, such as, for example, tobacco plants, for example by bathing wounded leaves or leaf sections in an agrobacterial suspension and subsequently growing them in suitable media. The transformation of plants by agrobacteria is known, inter alia, from F. F. White, Vectors for Gene Transfer in Higher Plants; in Transgenic Plants, Vol. 1, Engineering and Utilization, edited by S. D. Kung and R. Wu, Academic Press, 1993, pp. 15-38. Transgenic plants which comprise integrated into the expression cassette a gene for expressing a DXPRI gene can be regenerated in a known manner from the transformed cells of the wounded leaves or leaf sections.
To transform a host plant with a DNA encoding a DXPRI, an expression cassette is inserted into a recombinant vector whose vector DNA comprises additional functional regulatory signals, for example sequences for replication or integration. Suitable vectors are described, inter alia, in xe2x80x9cMethods in Plant Molecular Biology and Biotechnologyxe2x80x9d (CRC Press), chapter 6/7, pp. 71-119 (1993).
Using the above-cited recombination and cloning techniques, the expression cassettes can be cloned into suitable vectors which allow their multiplication, for example in E. coli. Suitable cloning vectors are, inter alia, pBR332, pUC series, M13mp series and pACYC184. Especially suitable are binary vectors which are capable of replication in E. coli and in agrobacteria.
The invention furthermore relates to the use of an expression cassette comprising the DNA sequence SEQ ID No. 1, or DNA sequences hybridizing herewith, for transforming plants, plant cells, plant tissues or parts of plants. The preferred object of the use is an elevated tocopherol, vitamin K, carotenoid, chlorophyll and polyterpene content of the plant.
Depending on the choice of promoter, expression may take place specifically in the leaves, in the seeds or in other parts of the plant. Such transgenic plants, their propagation material and the cells, tissues or parts of such plants are a further subject of the present invention.
In addition, the expression cassette may also be employed for transforming bacteria, cyanobacteria, yeasts, filamentous fungi and algae for the purpose of increasing the tocopherol, vitamin K, carotenoid, chlorophyll and polyterpene content.
The transfer of foreign genes into the genome of a plant is termed transformation. It exploits the above-described methods of transforming and regenerating plants from plant tissues or plant cells for transient or stable transformation. Suitable methods are protoplast transformation by polyethylene-glycol-induced DNA uptake, the biolistic method using the gene gunxe2x80x94the so-called particle bombardment method, electroporation, incubation of dry embryos in DNA-containing solution, microinjection and agrobacterium-mediated gene transfer. The abovementioned methods are described in, for example, B. Jenes et al., Techniques for Gene Transfer, in: Transgenic Plants, Vol. 1, Engineering and Utilization, edited by S. D. Kung and R. Wu, Academic Press (1993), 128-143, and in Potrykus, Annu. Rev. Plant Physiol. Plant Molec. Biol. 42 (1991), 205-225). The construct to be expressed is preferably cloned into a vector which is suitable for the transformation of Agrobacterium tumefaciens, for example pBin19 (Bevan et al., Nucl. Acids Res. 12 (1984), 8711).
Agrobacteria transformed with an expression cassette can equally be used in a known manner for transforming plants, in particular crop plants such as cereals, corn, oats, soya, rice, cotton, sugar beet, canola, sunflower, flax, hemp, potato, tobacco, tomato, oilseed rape, alfalfa, lettuce and the various tree, nut and grapevine species, for example by bathing wounded leaves or leaf sections in an agrobacterial suspension and subsequently growing them in suitable media.
Functionally equivalent sequences which encode a DXPRI gene are those sequences which still have the desired functions, despite a differing nucleotide sequence. Functional equivalents thus encompass naturally occurring variants of the sequences described herein, and synthetic nucleotide sequences, for example those obtained by chemical synthesis and adapted to suit the codon usage of a plant.
Functional equivalents are also to be understood as meaning, in particular, natural or artificial mutations of an originally isolated sequence which encodes a DXPRI which continue to show the desired function. Mutations encompass substitutions, additions, deletions, exchanges or insertions of one or more nucleotide residues. Thus, the present invention also encompasses, for example, those nucleotide sequences which are obtained by modifying the DXPRI nucleotide sequence. The purpose of such a modification may be, for example, the further limitation of the encoding sequence contained therein or else, for example, the insertion of further restriction enzyme cleavage sites.
Functional equivalents are also those variants whose function is attenuated or increased compared with the starting gene, or gene fragment.
Also suitable are artificial DNA sequences as long as they mediate the desired characteristic, for example an elevated tocopherol content in the plant, by overexpression of the DXPRI gene in crop plants, as described above. Such artificial DNA sequences can be identified, for example, by back translation of proteins with DXPRI activity which have been constructed by means of molecular modeling, or else by in vitro selection. Especially suitable are encoding DNA sequences which have been obtained by back translating a polypeptide sequence in accordance with the host-plant-specific codon usage. An expert skilled in the art of plant genetic methods will readily be able to identify the specific codon usage by computer evaluations of other known genes of the plant to be transformed.
Further suitable equivalent nucleic acid sequences which must be mentioned are sequences which encode fusion proteins, a DXPRI polypeptide or a functionally equivalent portion of these being a constituent of the fusion protein. The second part of the fusion protein may be, for example, another enzymatically active polypeptide, or an antigenic polypeptide sequence with the aid of which detection of DXPRI expression is possible (for example myc-tag or his-tag). However, it is preferably a regulatory protein sequence such as, for example, a signal or transit peptide which leads the DXPRI protein to the desired site of action.
An elevated tocopherol, vitamin K, chlorophyll, carotenoid and polyterpene content is to be understood as meaning for the purposes of the present invention the artificially acquired capability of an increased biosynthetic performance regarding these compounds by functional overexpression of the DXPRI gene in the plant in comparison with the non-genetically-modified plant for at least one plant generation.
The tocopherol biosynthesis site, for example, is generally the leaf tissue, so that leaf-specific expression of the DXPRI gene is meaningful. However, it is obvious that tocopherol biosynthesis need not be limited to the leaf tissue but may also take place in a tissue-specific fashion in the other remaining parts of the plant, for example in fatty seeds.
The constitutive expression of the exogenous DXPRI gene is also advantageous. On the other hand, inducible expression may also be desirable.
The expression efficacy of the transgenically expressed DXPRI gene can be determined, for example, in vitro by shoot meristem propagation. In addition, altered expression of the DXPRI gene with regard to type and level, and its effect on tocopherol biosynthesis performance may be tested on test plants in greenhouse experiments.
The invention furthermore relates to transgenic plants, transformed with an expression cassette comprising the sequence SEQ ID No. 1 or DNA sequences hybridizing herewith, and to transgenic cells, tissues, parts and propagation material of such plants. Especially preferred are transgenic crop plants such as, for example, barley, wheat, rye, corn, oats, soya, rice, cotton, sugar beet, canola, sunflower, flax, hemp, potato, tobacco, tomato, oilseed rape, alfalfa, lettuce and the various tree, nut and grapevine species.
Plants for the purposes of the invention are mono- and dicotyledonous plants, or algae.
Since this biosynthetic pathway is a metabolic pathway which is exclusively located in the chloroplasts, it offers optimal target enzymes for the development of inhibitors. Since, according to current knowledge, no enzyme which is identical with, or similar to, Arabidopsis thaliana DXPRI is present in other higher organisms, it can be assumed that inhibitors should have a very specific action on plants. The site of action of an inhibitor, namely fosmidomycin (3-(N-formyl-N-hydroxyamino)propylphosphonic acid; Fujisawa Pharmaceutical Co.) was identified as being a DXPRI. The biochemical assay shows efficient inhibition of the enzymatic activity (FIG. 7). The following abbreviations were used in FIG. 7: DOX=1-deoxy-D-xylulose, ME=methylerythritol. The same action is found in a plant assay in which barley seedlings are subjected to fosmidomycin treatment and then examined for their chlorophyll and carotenoid content. Both substances, which are derived from precursors of the isoprenoid metabolism, are greatly reduced in terms of quantity (FIG. 8).
Overexpression of the DXPRI-encoding gene sequence SEQ ID No. 1 and SEQ ID No. 3 in a plant allows, in principle, an improved resistance to DXPRI inhibitors to be achieved. Transgenic plants generated thus are also subject-matter of the invention.
Other subject-matters of the invention are:
Methods of transforming a plant, which comprise introducing, into a plant cell, into callus tissue, an entire plant or plant protoplasts, expression cassettes comprising a DNA sequence SEQ ID No. 1 or DNA sequences hybridizing herewith.
The use of the expression cassette comprising a DNA sequence SEQ ID No. 1 or DNA sequences hybridizing herewith for generating plants with an elevated resistance to DXPRI inhibitors by stronger expression of the DNA sequence SEQ ID No. 1, or DNA sequences hybridizing herewith.
The use of the DNA sequence SEQ ID No. 1 or DNA sequences hybridizing herewith for the generation of plants with an elevated tocopherol, vitamin K, chlorophyll, carotenoid and polyterpene content by expressing a DXPRI DNA sequence in plants.
The invention is now illustrated by the examples which follow, but not limited thereto:
The cloning steps carried out within the scope of the present invention, e.g. restriction cleavages, agarose gel electrophoresis, purification of DNA fragments, transfer of nucleic acids onto nitrocellulose and nylon membranes, linking DNA fragments, transformation of E. coli cells, growing bacteria, multiplying phages and sequence analysis of recombinant DNA, were carried out as described by Sambrook et al. (1989) Cold Spring Harbor Laboratory Press; ISBN 0-87969-309-6.
The bacterial strains used hereinbelow (E. coli, XL-I Blue) were obtained from Stratagene. The agrobacterial strain used for the transformation of plants (Agrobacterium tumefaciens; C58C1 with plasmid pGV2260 or pGV3850kan) was described by Deblaere et al. in Nucl. Acids Res. 13 (1985), 4777. Alternatively, the agrobacterial strain LBA4404 (Clontech) or other suitable strains may also be employed. Vectors which can be used for cloning are the vectors pUC19 (Yanish-Perron, Gene 33 (1985), 103-119) pBluescript SK- (Stratagene), pGEM-T (Promega), pZerO (Invitrogen), pBin19 (Bevan et al., Nucl. Acids Res. 12 (1984), 8711-8720) and pBinAR. (Hxc3x6fgen and Willmitzer, Plant Science 66 (1990), 221-230).
Recombinant DNA molecules were sequenced using a laser fluorescence DNA sequencer by Licor (available from MWG Biotech, Ebersbach) following the method of Sanger (Sanger et al., Proc. Natl. Acad. Sci. USA 74 (1977), 5463-5467).